As used herein, “/” denotes alternative names for the same or similar components or structures. That is, a “/” can be taken as meaning “or” as used herein. Unicast transmissions are between a single sender/transmitter and a single receiver. Broadcast transmissions are between a single sender/transmitter and all receivers within receiving range of the transmitter. Multicast transmissions are between a single sender/transmitter and a subset of the receivers within receiving range of the transmitter where the subset of receivers with receiving range of the transmitter may be the entire subset. That is, multicast may include broadcast and is therefore a broader term than broadcast as used herein. Data is transmitted in packets or frames.
In wireless local area networks, an access point (AP)/base station/station (STA)/mobile device/mobile terminal/node transmits multicast and broadcast frames immediately after it determines that the wireless medium/channel is idle/clear using a physical and virtual carrier sense process with a random backoff time. Therefore, collisions may occur when multiple STAs attempt to transmit simultaneously. For example, an access point and its associated STAs may transmit simultaneously and then all the transmitted frames are lost due to collision. For example, when an AP sends multicast data frames to several of its associated STAs, another STA may send its data or control frames to the AP. The AP with its associated stations is called a basic service set (BSS). Collisions occurring within a BSS are called intra-BSS collisions. In another example, AP1 and AP2 operate on the same channel/frequency and the two BSSs overlap. AP1 transmits data frames to its associated STAs. However, AP2 cannot hear AP1's transmission and thinks that the medium is idle. AP1 is a “hidden node” to AP2. AP2 may transmit frames to its associated STAs while AP1 is transmitting. But there are STAs that are in the interference range of both AP1 and AP2 so that the transmitted frames from AP1 and AP2 are lost at the STAs due to collision. This type of collision is called overlap BSS collision or inter-BSS collision.
Multicast and broadcast provide an efficient method to distribute data to multiple receivers. However, it is a challenge to ensure the reliable receipt of data by multiple receivers in multicast and broadcast. It is especially important to provide an efficient mechanism for acknowledging multicast packets received by multiple receivers at the media access control and link layer.
Higher-layer (application layer and transport layer) schemes have been proposed to improve multicast reliability. In one prior art scheme, a negative acknowledgement (NACK) method was described, in which a receiver sends a NACK to the sender when it detects a gap of sequence number in the received packets. This approach results in additional delay and buffer requirements at both senders and receivers. Although the feedback overhead may be reduced by suppressing the number of NACKs sent by multiple multicast receivers, this scheme increases the complexity of guaranteeing high multicast reliability or no packet loss.
US2006/0109859 A1 “METHOD AND SYSTEM FOR EFFICIENT AND RELIABLE MAC-LAYER MULTICAST IN WIRELESS TRANSMISSIONS” by Acharya et al. describes a media access control (MAC) layer method for reliable multicasting in a wireless network. A sender node sends a Request-to-Send (RTS) signal to the receiver nodes. The RTS signal includes a set of identifiers or a bit vector corresponding to the receiver nodes. Each bit in the bit-vector indicates a recipient node. Clear-to-Send (CTS) signals are sent from the receiver nodes according to an order function of the set of identifiers or the bit vector. The sender sends a data packet to the receiver nodes after receiving at least one CTS signal. One or more ACK signals are sent from one or more of the receiver nodes according to an ordering function of the set of identifiers or a bit vector in the RTS signal, if a data packet has been received correctly. This approach uses a four-way handshake to send and to acknowledge a single data packet. The order of the ACK signals from multiple receivers is based on the identifiers or a bit vector in the RTS signal. The ACK signal acknowledges only a single packet. This scheme incurs a high overhead and hence reduces the network throughput.
In addition, in a wireless local area network (WLAN), an access point (AP) may have a large number of associated stations, or a sending node may have a large number of neighbor nodes. If the RTS signal includes a set of identifiers corresponding to the receiver nodes or a bit-vector, the RTS signal may become large, leading to more overhead.
Another approach uses multiple unicasts to send multicast data in a wireless network. An access point repeatedly unicasts multicast data packets, each unicast dedicated to a single intended receiver because unicast provides MAC layer acknowledgement and retransmission in the wireless network environment. However, this approach requires transmitting the same multicast data multiple times over a common broadcast channel, leading to a high overhead and low network throughput.
WO 2008/020731 A1 “MULTICAST PROCEDURE IN A WIRELESS NETWORK” by Huh et al., describes a method in which an originator/access point transmits data packets in multicast to multiple receivers and then uses unicast to poll each of the receivers individually for its receiving status of previously transmitted multicast data packets. Because the originator sends the block acknowledgement request (BAR) message in unicast to each of the receivers to request the receiving status individually, it transmits the BAR messages multiple times, one for each receiver. That is, the originator exchanges in unicast the block acknowledgement request (BAR) message and acknowledgement reply (B-ACK) messages with each of the receivers individually, one for each receiver. Therefore, this approach still incurs unnecessary overhead and reduces network efficiency and throughput.
It would be advantageous to have a method and apparatus to acknowledge multiple multicast packets from multiple receivers at the MAC-layer in wireless network environment. It is desirable to have an efficient method for reliable multicast data transmission, acknowledgement and retransmission.